Director since 1981 of NASA's Goddard Institute for Space Studies, Hansen made headlines worldwide in 1988 with his US Congress testimony that climate change was already well under way, a finding highly contested at the time.

Hansen believes only an abrupt and profound change in the way we consume energy can stop the rapid rising of greenhouse emissions in time, and he is highly sceptical that a global emissions trading system will bring this about.

"It takes about 10 years to negotiate it and get all the countries on board, and then you make all sort of compromises, so it turns to be very ineffectual," he said.

He says the concentration of CO2 needs to be kept under 350 parts per million.

A cap-and-trade system, under which "polluting rights" are exchanged in a carbon market, is likely to be reinforced at United Nations climate change conference in Copenhagen this December as the favoured approach to slashing greenhouse gases.

Hansen calls instead for a direct tax - as close to the source as possible - on fossil fuels, which he says would enable an international agreement to be globally effective in a short period of time.

"If it's going to be cap-and-trade, I'd rather nothing came out of Copenhagen. I'd rather take another year and two and get it right," he said.

Australian scheme

Minister for Climate Change and Water, Senator Penny Wong, this week released draft legislation for Australia's emissions trading scheme, called the Carbon Pollution Reduction Scheme (CPRS), amid criticism from various political parties.

The scheme, set to start in 2010, would see Australia cut emissions by 5 to 15% before 2020, based on 2000 levels.

Environmentalists say the cap on carbon is too high, and big polluters are getting too much compensation.

Industry says the CPRS should be delayed until 2012, and "Emissions Intensive Trade Exposed (EITE) industries" should be better protected so jobs and business investment does not go offshore.

Support for Hansen

Some Australian experts support Hansen's views, in the light of what is proposed for the CPRS.

Richard Denniss of the Canberra-based Australia Institute says the CPRS is "seriously flawed" with "pathetically low targets and far too much money diverted to compensation".

The European Union is expected to push for a 40% reduction at Copenhagen in December.

"The main problem with emissions trading is that if you get the targets wrong you are locked into failure," says Denniss.

He says, by comparison, a carbon tax can be increased once it is introduced, and also enables greater reductions of emissions.

Denniss says under a cap-and-trade system, savings in emissions in one area simply allow more emissions in another. Investment in public transport or household energy efficiency does nothing to reduce emissions below the cap.

But, he says, under a carbon tax, such investment will enable emissions to be cut beyond the cap.

["If there is a carbon tax in place and I put a solar panel on my roof no one else has an incentive to run out and take their's off," says Denniss.

A back-up plan

Melbourne-based sustainable energy consultant Alan Pears is also concerned about the CPRS "locking in failure".

The CPRS must have criteria that allow government to tighten the cap with 12 months notice in response to a low carbon price or scientific evidence supporting stronger targets, says Pears, an adjunct professor at RMIT University, Melbourne.

Pears says he believes Australian businesses are more likely to innovate under an emissions trading scheme than a tax, because it would signal a business opportunity rather than red tape that requires minimum compliance.

But, he wants a lower cap, requirements for big polluters to pay back their compensation if they make a windfall profit from it, and says the CPRS should take account voluntary cuts in emissions.

"If we can't get a good trading scheme by [mid-2010] we should introduce a carbon tax as an interim measure, with the aim of a trading scheme [as soon as possible]," says Pears.

Politically "naive"

Others believe it's best to focus on making the CPRS more stringent.

"I've got serious question marks over shelving the whole [emissions trading] process and going for a carbon tax," says John Connor of the Climate Institute.

Connor, and others, reject Hansen's assumption a carbon tax can be adopted more rapidly than the cap-and-trade schemes currently under development.

"If he thinks that a carbon tax will be an easier political sell I think that's naïve," says Connor.